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Exercise during pregnancy may increase birth weight

BMJ 2001; 322 doi: https://doi.org/10.1136/bmj.322.7280.193/a (Published 27 January 2001) Cite this as: BMJ 2001;322:193
  1. Scott Gottlieb
  1. New York

    Women who start a moderate exercise programme during early pregnancy may improve their likelihood of giving birth to a healthy baby, according to the results of a new study.

    Dr James Clapp, professor of reproductive biology at Case Western Reserve University in Cleveland, Ohio, and colleagues studied 46 women who were not exercising regularly and were 8 weeks pregnant. The women were randomly asked to undertake either moderate weight bearing exercise three to five times a week (n=22) or no exercise for the rest of the pregnancy (n=24) (American Journal of Obstetrics and Gynecology 2000;183:1484-8).

    Infants born to the women who exercised were significantly heavier and longer than those whose mothers did not exercise. In addition, mid-trimester placental growth rate was faster and indexes of placental function were greater in the exercise group. No significant differences between the two groups were found in neonatal percentage body fat, head circumference, ponderal index, or maternal weight gain.

    The findings may be especially useful for women at risk of having low birthweight babies—such as older women and those who gain little weight during pregnancy. An exercise programme may be an easy, acceptable way to boost the growth of the fetus in normal pregnancy, the researchers said.

    Exercise, in fact, may seem easier during pregnancy. Pregnancy induces a “marked training effect,” Dr Clapp said. During pregnancy the heart pumps more blood and the lungs take in more air. Contrary to persisting assumptions, he said, exercise does not induce early labour, and it reduces fetal distress.

    The American College of Obstetrics and Gynecologists used to recommend that expectant mothers keep their pulse rate no higher than 140 during exercise. Now the college says that maximum heart rates should be determined individually, depending on a woman's age and fitness level. The college advises women to avoid exercise in supine positions after the first trimester because it directs blood flow away from the baby.

    Dr Clapp defines a regular exercise as engaging in a weight bearing activity, such as jogging or aerobic dancing, for 30 continuous minutes three or more days a week at or above 65% of aerobic capacity.

    Because the current study was conducted in low risk, healthy women, the findings do not necessarily indicate that the exercise will be as beneficial in high risk or unhealthy women. “Nonetheless,” the authors wrote, “early introduction of a moderate-intensity regimen of weight-bearing exercise during pregnancy may have preventive value in individuals or populations at risk of having low birth weight newborns.”

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